Metal Finishing Guide Book


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Page 81 of 903

THE AQUEOUS PROCESS Most aqueous processes require, as a minimum, a wash, a rinse, and a dry. There may be multiple wash and rinse stages, depending upon the desired cleanliness level. Typically, wash requires a mild alkaline cleaner that is heated. Care should be taken in the selection of the cleaner Fig. 5. Five-tank vertical agitation and to ensure that it is compatible with rotation system with material handling and alloys being cleaned and has the ability closed-loop wastewater treatment system. to emulsify or split oils and remove othSystem utilizes a wash,ultrasonic wash, er contaminants. three counterflow rinses, and rotation of The purpose of rinses is to remove baskets in a recirculation hot-air dryer. residual chemistry remaining on parts. If only one rinse is employed, a flow of clean water into the rinse tank on a continual basis to an overflow dam is necessary to maintain rinse clarity. A system employing two rinses should consume half the water that a single-rinse system will require. Likewise, three rinses will utilize half the water that a two-rinse system will use to maintain final rinse tank clarity. Depending upon cleanliness specification and whether spot-free drying is necessary, RO or DI waters may be necessary for final rinses. If RO or DI water is employed consideration should be made to prevent flash rusting on ferrous components. It may be necessary to add rust inhibitors or wetting agents to rinse tanks to prevent flash rusting while rinsing ferrous components. WATER QUALITY If incoming municipal waters contain a high level of chlorine, sulfur, salts, or other contaminants it may be necessary to treat this water prior to use. Water hardness and poor quality of water can affect the ability of the aqueous chemistries to work. Likewise, poor water quality can affect drying and cause water spotting. SYSTEM CONSTRUCTION All aqueous cleaning systems should be constructed with stainless steel for tanks and wetted surfaces. Whereas mild steel construction was acceptable in the past the greatest cost factor in manufacturing an aqueous system is labor. Stainless steel, although three times the cost of mild steel, should have a cost impact of less than 20% on the overall system. Mild steel construction will always oxidize in time and cause degradation of baths, compromise the ability to clean, and cause rinsewater to always Fig. 6. Vertical agitation and rotation contain ferrous oxides. Stainless steel consystem with access doors open. System struction overcomes these shortcomings agitates baskets 24 in. X 24 in. X 12 in. and should give an aqueous system a minthrough a prewash, wash, and two imal 20-year life. counterflow rinses prior to dry. 78

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